Former Yahoo Chief Executive Marissa Mayer apologized on Wednesday for a pair of massive data breaches at the internet company and blamed Russian agents at a hearing on the growing number of incidents involving major U.S. companies.

"As CEO, these thefts occurred during my tenure, and I want to sincerely apologize to each and every one of our users," she told the Senate Commerce Committee, testifying alongside the interim and former CEOs of Equifax and a senior Verizon executive.

"Unfortunately, while all our measures helped Yahoo successfully defend against the barrage of attacks by both private and state-sponsored hackers, Russian agents intruded on our systems and stole our users' data."

Verizon, the largest U.S. wireless operator, acquired most of Yahoo's assets in June, the same month Mayer stepped down. Verizon disclosed last month that a 2013 Yahoo data breach affected all 3 billion of its accounts, compared with an estimate of more than 1 billion disclosed in December.